1. Field
Certain exemplary embodiments of the present invention relate generally to wireless communications, and more particularly, to a method and a base station for a cell selection.
2. Description of the Related Art
Various abbreviations that appear in the specification and/or in the drawing figures are defined as below:    3GPP 3rd Generation Partnership Project    BS Base Station    BSR Buffer Status Report    DL Downlink    eNB evolved Node B    HetNet Heterogeneous Network    IE Information Element    LTE Long Term Evolution    MAC Media Access Control    RRC Radio Resource Control    RSRP Reference Signal Received Power    SNR Signal to Noise Ratio    UE User Equipment    UL Uplink
A demand for higher data rates in wireless networks has been unrelenting and triggered the design and development of new data-minded cellular standards, such as a 3GPP LTE standard. Currently, such a rapidly increasing demand is fulfilled mainly by higher bandwidth allocations. Since the bandwidth has been scarce and expensive thus far, the crux of substantial throughput enhancements may be ascribed to improvements of the reuse of radio frequency resources. In this regard, cells with relatively small coverage areas may achieve the efficient spatial reuse of spectrum.
In light of the above, HetNet has been proposed and investigated by the 3GPP as part of a Study Item for LTE-Advanced (LTE-A) to provide better spectrum efficiency and enable high performance for user in hot spots, which are generally covered by low power BSs or nodes as discussed below. HetNet deployment is defined as a mixed deployment consisting of high power BSs (e.g., macro BSs) and low power BSs. Such low power BSs, including but not limited to pico BSs, femto BSs and relay nodes, have been introduced into LTE-A systems with targets of improving a system capacity, extending the serving coverage to cover hot zones, and serving users in coverage holes and so on. Such low power BSs may also provide true cell-splitting gains in the HetNet for carrying new frequency-time resources.
Although UEs may notably benefit from the above HetNet, a UE at a cell edge is less likely to have best DL and UL performance concurrently with its serving cell. The root cause behind this is that, due to the introduction of the low power BSs, traditional cell selection techniques would result in serious UL and DL imbalance, which would degrade over all system performance.
In particular, under the existing wireless systems including LTE Rel 8, the UE, from a DL point of view, would connect to a cell (i.e., a BS) that provides the highest DL received power among multiple cells, and that may be ascertained by using RSRP as a metric. Since there is a large degree of imbalance between the transmit power of a high power BS and that of a low power BS, and thereby the coverage area of the low power BS turns out to be much smaller than that of the high power BS, the UE would select the high power BS as its serving cell instead of the low power BS in terms of the cell range (i.e., coverage area). On the other hand, from a UL point of view, the selection of an optimal serving cell or BS is based upon the lowest path loss, which may be measured by the distance from the UE to the BS, rather than the highest DL received power as discussed above. Hence, in case the UE is closer to the low power BS than to the high power BS, the low power BS should be selected as a serving BS in terms of the path loss. For a better understanding of the above-mentioned circumstances, a discussion will be made with reference to FIG. 1.
FIG. 1 exemplarily illustrates a simplified HetNet 100 consisting of a high power BS 101, a low power BS 102 and a UE 103. Due to distinct transmit power, the coverage area of the high power BS 101, as depicted by a large ellipse, encompasses the coverage area of the low power BS 102, as depicted by a relatively smaller ellipse. Further, it can be seen from the FIG. 1 that the UE 103 is further away from the high power BS 101 than from the low power BS 102.
We posit a scenario that the UE 103 has a huge amount of UL traffic to transmit and needs to select a proper BS to proceed, which is very likely to take place in the practical communication. As discussed previously, the UE 103 should select the high power BS 101 as its serving BS once the DL received power is applied as a criterion for the cell selection. Conversely, the UE should select the low power BS 102 as its serving BS in terms of the link loss. However, under the present scenario, if the high power BS 101 is selected as the serving BS for the UE 103, the UL bitrates of the UE 103 would be subject to large losses due to poor UL quality. It is apparent that selection of a serving BS merely relying upon the RSRP or cell range is problematic and could be damaging to the upcoming UL traffic of the UE.
To address the potential problems as above, a fixed biased RSRP solution has been proposed in the standard 3GPP TS 36.321 to enable cell range expansion of the low power BS such that its likelihood of being selected as a serving BS could be increased in the HetNet. In particular, this cell selection scheme adds a bias value in the RSRP such that more UEs are able to select low power BSs as their own serving BSs through the following equation:CellID=argmax{i}{RSRP_i+bias_i},  (1)
wherein the CellID is an identifier of a cell, i denotes a BS number (e.g., from 1 to N), RSRP_i denotes the RSRP of the ith BS as measured by the UE, and bias_i is a fixed bias value (a.k.a., “bias RSRP”) with respect to the ith BS and is predefined during the network planning. By addition of a fixed bias value to the RSRP_i, the cell range of the low power BS appears to be extended during the cell selection procedure, and thus the low power BS is more likely to be selected as a serving BS according to the RSRP-based cell selection criterion. As a result, after the low power BS is selected as a serving BS, the UL signal strength may be improved remarkably due to less link losses, even though the DL signal strength might decrease a little. However, even with cell range expansion in combination with the fixed bias RSRP, the optimal tradeoff between the UL and DL performance still cannot be achieved because traffic demands for the UL and DL are varied over time while the fixed bias RSRP cannot be adjusted correspondingly and dynamically.
For example, due to a sufficiently big fixed bias RSRP, the UE 103 may select the low power BS 102 as its serving cell. In the mean time, the UE 103 may have a very high DL traffic demand and none or a very low UL traffic demand. In this case, decreasing the DL performance to offset the UL performance once the low power BS 102 is selected as a serving BS does not contribute to the tradeoff between the UL and DL traffic. As a further example, if the value of the fixed bias RSRP is insufficiently big, the UE 103 is still likely to select, according to the equation (1), the high power BS 101 as its serving BS even if the UE 103 has a very high UL traffic demand and none or a very low DL traffic demand. It is clear that the optimal tradeoff between the DL traffic and the UL traffic is not realized.
It can be seen from the above examples that the cell range expansion with the fixed bias RSRP cannot provide an optimal cell selection solution.